cxgb(4): Rework my commit 9dc7c250.

The previous implementation was reported to try to coalesce packets
in situations when it should not, that resulted in assertion later.
This implementation better checks the first packet of the chain for
the coallescing elligibility.

Approved by: re (gjb@)

(cherry picked from commit d510bf133d045d6c83742aeda6949bec150f6cbf)
(cherry picked from commit 1f3cff343b451d9fc9c10f4f8c4b416ed5eae23f)